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Former Republican senator Chuck Hagel of Nebraska, possibly President Obama's top choice for Defense secretary, is drawing fire for his positions on Israel, Iraq and gay rights.

The criticism, mostly from the decorated Vietnam veteran's former Republican Senate colleagues, signals a shift in the battle over Obama's second-term national security team from secretary of State to Defense.

Friday, Obama picked Sen. John Kerry, D-Mass., chairman of the Senate Foreign Relations Committee, to succeed Hillary Rodham Clinton as secretary of State. He's likely to easily win confirmation after United Nations Ambassador Susan Rice withdrew from consideration amid sharp criticism from Senate Republicans over her comments after the attack on the U.S. consulate Sept. 11 in Benghazi, Libya.

The concerns raised about Hagel, 66, have been voiced far less vehemently, but they probably foreshadow some rough sledding in the Senate if Obama picks him for the Pentagon's top post. Defense Secretary Leon Panetta has indicated he plans to step down early next year.

"I think it would be a challenging nomination," Sen. Lindsey Graham, R-S.C., said Sunday on NBC's Meet the Press.

Sen. Johnny Isakson, R-Ga., lauded Kerry on ABC's This Week but was tight-mouthed about Hagel: "I served with him in the Senate, but he hasn't been nominated yet," Isakson said. "I'll reserve any judgment until after the hearings we have on confirmation if, in fact, he is nominated."

Sen. Joe Lieberman, I-Conn., predicted a "very tough confirmation process" on CNN's State of The Union.

Some Democrats have been more supportive. Michigan's Carl Levin, chairman of the Senate Armed Services Committee, said Hagel would be "terrific." Sen. Charles Schumer, D-N.Y., simply told NBC, "I'd have to study his record."

Hagel, a 12-year senator who retired in 2009, was often a thorn in his own party's side. Although he supported the Iraq War, Hagel, who was wounded in Vietnam, assailed President George W. Bush's plan to send 30,000 additional troops to Iraq as "the most dangerous foreign policy blunder in this country since Vietnam, if it's carried out."

He also backed the Afghanistan War but has grown increasingly dubious of the more-than-decade-old U.S. involvement in the conflict. In a radio interview this year, he advocated more diplomacy and said "the American people want out" of the war.

Hagel has been critical of a possible military strike by the United States or Israel against Iran.

"The appointment of Chuck Hagel would be a slap in the face for every American who is concerned about the safety of Israel," said Matt Brooks, executive director of the Republican Jewish Coalition.

Hagel is also in the hot seat for remarks he made in 1998 about James Hormel, an openly gay nominee for an ambassadorship. In an interview with the Omaha World-Herald in 1998, Hagel said that for a U.S. ambassador, "it is an inhibiting factor to be gay." He went on to call Hormel "openly, aggressively gay."

Thursday, Chad Griffin, president of the Human Rights Campaign, called Hagel's comments and votes on gay issues "unacceptable."

"The next secretary of Defense must be supportive of open service as well as equal benefits for lesbian and gay military families, and Sen. Hagel must address these issues immediately," Griffin said.

Friday, Hagel apologized for his remarks in a brief written statement, saying they were "insensitive" and did not reflect his views. He said he supports "open service."

When he took office in 2009, Obama asked Republican Robert Gates to remain as Defense secretary. Both Hagel and Gates have spoken of the need for global answers to regional conflicts and emphasized the use of economic and political aid to bolster weak nations. Obama and Hagel served together in the Senate, developing close ties.

As Defense secretary, Hagel would oversee the withdrawal of troops from Afghanistan and steep cuts in Pentagon spending, as well as an increased focus on regional challenges from Syria, Iran and North Korea.